1. Field of the Invention
The present invention relates to evaluation of subsurface earth formations, more particularly the detection of fractures in them.
2. Description of Prior Art
The determination of in-situ fracture orientation and fracture density in earth formations can help optimize hydrocarbon production from them. Knowledge of fracture orientation and density also provides information for horizontal drilling programs. It can also be used to evaluate the hydraulically induced fractures encountered during enhanced recovery processes.
Vertical fractures of the rocks cause an azimuthal anisotropy for shear-wave propagation. In vertical fractures of rock, the velocity of a vertically-travelling shear wave depends on whether the wave polarization is perpendicular or parallel to the direction of fracturing. Two shear waves with different speeds are known to occur. Since the wave speeds are different, a splitting of the shear waves has been observable.
Based on the principle of shear-wave splitting, two seismic methods have been used to determine the orientation of vertical fractures in the subsurface: 1) the shear-wave reflection method, and 2) the shear-wave vertical seismic profile, or VSP, method. Examples of these methods are in U.S. Pat. Nos. 4,803,666; 4,903,244; 4,817,061; 4,888,743 and 4,933,913. The reflection method has both the shear-wave sources and receivers placed on the earth surface. The VSP method is based on seismic receivers being placed in a well and the sources placed on the surface. In both cases, four-component shear-wave traces are recorded.
The data obtained from these seismic methods has typically been processed according to a rotation technique disclosed by Alford, "Shear Data in the Presence of Azimuthal Anisotropy," 56th Ann. Internat. Mtg., Soc. Explor. Geophys., Expanded Abstract, 476-479. The rotation method was applied to these traces to find the principal polarization planes of the shear waves. The fracture orientation was determined by comparing shear-wave traveltimes along the two polarization planes. The plane with the shorter traveltime is coplanar with the vertical fractures, and thus indicates the orientation of the vertical fractures.
The orientation obtained with either the surface seismic or VSP method was the "average orientation," because it represented the composite anisotropic property of the formation materials within the entire interval between the surface and the recording depth level. To be geologically meaningful, the results had to be transformed into "interval orientation" to represent the in-situ fracture orientation. This had to be done through a "layer-stripping" procedure proposed by Winterstein and Meadows. "Changes in Shear-wave polarization azimuth with depth in Cymric and Railroad Gap Oil Fields," Expanded Abstracts, 60th SEG Meeting, San Francisco 1435-1438.
There were several problems with these earlier methods. For example, both reflection and VSP shear wave data acquisition were relatively complex and expensive procedures. Further, the VSP shear wave method was limited to land seismic operations. In addition, the "layer stripping" data processing technique often was subject to large uncertainties in the processing results. Poor resolution was also a frequent problem.